In the conventional production of internal combustion engines for automobiles or motorcycles, cylinders have been made mostly from gray cast iron which is inexpensive and excellent in wear resistance and seizing resistance, and partly from cast aluminum alloy plated with chrome or some other metal. In recent years, however, considerable effort has been directed to decreasing the weight of engine parts to produce more fuel-efficient automobiles or motorcycles. With such a technical trend, even cylinder liners, which should have good heat resistance and wear resistance, are now required to be lighter in weight. For example, a cylinder liner made of an aluminum alloy has been proposed in JP-A-52-109415. (The term "JP-A" as used herein means an "unexamined published Japanese patent application No.".)
Further, JP-A-59-59855 and JP-A-59-59856 propose aluminum alloy shapes which are obtained by subjecting aluminum alloy powder having a specific composition to hot extrusion to provide products which can be used for various applications.
However, the aluminum alloys which have so far been proposed have some disadvantages when they are used as a cylinder liner for an internal combustion engine. Namely, cylinder liners are required to have good wear and heat resistance and be compatible with pistons. An aluminum alloy practically employable as a cylinder liner has not yet been obtainable. By way of illustration, cylinder liners made of such produced aluminum alloys are defective, as compared with conventionally known cylinder liners, in that they show a lower seizing load measured in the absence of a lubricating oil or they are more prone to attack pistons although the wear resistance of such aluminum alloy cylinder liners is almost comparable to that of conventional ones. These problems have not yet been solved.
In an attempt to solve the above problems, there has been proposed, for example, an alloy prepared by incorporating a hard material or a lubricating material into an Al-Si alloy so as to improve its insufficient wear resistance and the sliding or moving properties of the alloy. However, the incorporation of these additives causes problems with the processing and machining of the alloy. That is, in the case of an alloy in which a hard material has been incorporated, there is a great difficulty in processing the alloy. In the case where carbon, for example, is incorporated as a lubricating material into the alloy and the resulting alloy is shaped into a cylinder, there is the problem that graphite particles are linearly dispersed at the inner periphery of the cylinder in the direction of its length and this causes the radial crush strength of the alloy cylinder to be low.
The Al-Si alloys have been regarded as a hopeful material for cylinder liners because they have a smaller thermal expansion coefficient than aluminum due to the addition of Si and because their weight is low. However, if aluminum oxide or silicon carbide is incorporated as a hard material into the Al-Si alloy in order to further improve its wear resistance, the machinability of the resulting alloy becomes very poor, although the wear resistance of cylinder liners obtained from the alloy is improved.